The homeobox gene Hb9 is expressed selectively by motor neurons (MNs) in the developing CNS. Previous studies have identified a 9 kb 5′ fragment of the mouse Hb9 gene that is sufficient to direct gene expression to spinal MNs in vivo.
In the vertebrate CNS, the specification of neural identity is initiated by humoral inductive factors, that impose a specific profile of transcription factor expression on neural progenitor cells, thereby restricting their phenotypic differentiation (Jessell et al., Cell 68:257-70 (1992), Goridis et al., Curr Opin Neurobiol 9:47-53 (1999)). In the developing spinal neuroepithelium, motor neuron (MN) progenitors arise in part in response to the ventralizing action of Sonic hedgehog (Shh) (Briscoe et al., Curr Opin Neurobiol 11:43-9 (2001)). The specification of MN progenitors by Shh is mediated through the patterned expression of homeodomain (HD) and basic helix-loop-helix (bHLH) transcription factors; these function primarily as transcriptional repressors (Muhr et al., Cell 104:861-73 (2001)), whose cross-regulatory interactions establish distinct progenitor domains (Briscoe et al., Nature 398:622-7 (1999); Briscoe et al., Cell 101:435-45 (2000); Jessell, Nat Rev Genet 1:20-9 (2000); Vallstedt et al., Neuron 31:743-55 (2001)). Through this general scheme, MN progenitors are restricted to a narrow region of the ventral neural tube termed the pMN domain (Briscoe et al., Cell 101:435-45. (2000); Jessell, Nat Rev Genet 1:20-9 (2000); Pierani et al., Neuron 29:367-84 (2001)) Within this domain, MN progenitors are characterized by two HD proteins, Nkx6.1 and Pax6, and a bHLH protein, Olig2. Together, these proteins serve to initiate the expression of distinct MN transcription factors, which include the HD protein HB9.
HB9 is expressed selectively by post-mitotic spinal MNs in the developing vertebrate CNS, and serves as a marker for the MN phenotype (Tanabe et al., Cell 95:67-80 (1998), Arber et al., Neuron 23:659-74 (1999)). Genetic studies in mice have suggested its importance in the consolidation and maintenance of MN identity (Arber et al., Neuron 23:659-74 (1999), Thaler et al., Neuron 23:675-87 (1999)). A 5′ 9 kb Hb9 promoter has been shown to drive MN-specific expression in vivo (Arber et al., Neuron 23:659-74 (1999), Wichterle et al., Cell 110:385-397 (2002)). Nonetheless, the regulatory control of Hb9 gene expression is only poorly understood.
The present invention is directed to overcoming this deficiency in the art.